The present invention relates to a device used in an automatic and continuous method of converting heavy metal hydroxides, which are an environmental hazard and are poisonous, and which are obtained in sludge from purification works for pickling, rinsing and surface-treatment plants, into heavy metal oxides in powder and/or granulated form.
There are many industrial processes for which fresh water is required, where the water, after having been used, contains various pollutants that are carried along with the waste water into the ocean, lakes, rivers and streams, unless they pass first through municipal purification workds. In special cases of pollution, it is inappropriate to allow the waste water to be fed into municipal purification works. This applies particularly when the industrial waste contains heavy metals such as lead, copper, chromium, nickel, cadmium, zinc, etc.
The metals most often occur together with acids, and then form very poisonous compounds. The compounds occur in pickling and surface-treatment processes which require for the subsequent rinsing processes much fresh water and result in highly polluted waste water.
If suitable acid and metal recovery devices are not available, the used bath water is discharged into the sewer system together with the wash water.
Industries are now required, to an increasing degree, to arrange for waste water treatment before the waste water is discharged into the municipal purification works or out into rivers and streams or other open waters.
This treatment usually consists of neutralizing the acids with the aid of basic additives and with the aid of special additives (flocculation agents) to precipitate the dissolved metals in hydroxide flocks, which are either allowed to sediment in decantering vessels or are skimmed off in floatation plants, whereupon the treated water is harmless for discharge into the municipal purification works or into open water.
The hydroxide sludge, which contains 96-98% water, is still poisonous and constitutes an environmental hazard, and as simple methods of destroying the sludge are not available in the matket at present, the companies have great difficulty in disposing of it. Central collection places have been arranged, to which the sludge is conveyed with tank trucks. There are, of course, risks involved in transporting considerable quantities of poison on public roads. Moreover, it is unsatisfactory that large quantities of poison are concentrated in certain places, as the risk for unintentional discharging through leakage increases as the stocks grow continuously since there is no suitable destruction equipment available.
From an environmental point of view it would be most appropriate to neutralize dangerous poisons and waste at the source, so that the residual products will be entirely harmless and, if possible, can either be subjected to recovery processes or else can be deposited among ordinary municipal waste.
In U.S. Pat. No. 3,960,725, there is set forth a method of achieving suitable and safe disposal of these wastes by first forming the sludge into a thixotropic consistency and then applying this thixotropic sludge in a film-like manner onto a rotating drum carrier where the film is dried and then formed into metal hydroxide powder or granules which are then collected, concentrated and sintered to form harmless metal oxides which are extremely difficult to dissolve, thereby rendering them safe.